There are numerous fields in which a particulate feed material must be uniformly distributed and introduced into a device, with respect to both time and space, while maintaining a well-mixed and steady particle size distribution within the feed stream.
A group of such applications concerns the delivery of particulate materials such as pulverized coal, dust or combustible ores to a combustion system, such as may be found in burners for heat generation, insufflation or smelting.
One such application that requires uniformity of feed flow is flash smelting for sulphide concentrates, such as may be encountered in the production of copper, nickel, lead or zinc. A flash smelting furnace typically includes an elevated reaction shaft at the top of which is positioned a burner or multiple burners where particulate feed material and reaction gas are brought together. In the case of copper smelting, the feed material is typically an ore concentrate containing both copper and iron sulfide minerals. The concentrate is usually mixed with a silica flux and combusted with pre-heated air or oxygen-enriched air. Molten droplets are formed in the reaction shaft and fall to the hearth, forming a copper-rich matte and an iron-rich slag layer.
A conventional burner for a flash smelter includes an injector having a water-cooled sleeve and an internal central lance, a windbox, and a cooling block that integrates with the roof of the furnace reaction shaft. The lower portion of the injector sleeve and the inner edge of the cooling block create an annular channel. Oxygen enriched combustion air enters the windbox and is discharged to the reaction shaft through this annular channel. The water-cooled sleeve and the internal lance of the injector also create an annular channel within the combustion air flow annulus. The feed material is introduced from above and descends through the injector sleeve into the reaction shaft through this internal annulus. Deflection of the feed material into the reaction gas is promoted by a bell-shaped tip at the lower end of the central lance. In addition, the tip includes multiple perforation jets that direct compressed air outwardly to disperse the feed material in an umbrella-shaped reaction zone. Such a burner for a flash smelting furnace is disclosed in U.S. Pat. No. 6,238,457.
The material feed supply equipment is typically comprised of bins and hoppers, mechanical feeders, conveyors, splitter boxes, manifold connectors, and feed pipes located above the injector. Typical feeders and conveyors include screw-feeders, table feeders, drag-chain conveyors and air slides. Some feed systems also combine feed streams of different particle density, shape, and size upstream of the burner.
Known feed systems of this type are associated with disadvantages that can adversely affect the burner performance and cause problems, such as: poor oxygen efficiency; variable furnace metallurgy and matte grade; increased copper losses to slag; increased elutriation of dust to the off-gas handling equipment, etc. These problems result from a failure to achieve uniformity of the feed material both spatially and in time on the appropriate scales, as well as causing segregation of the individual feed components with respect to particle size, density and/or shape.
For example, it is well documented that known mechanical feed systems, such as drag chains, screw conveyors and table feeders deliver the feed in discrete packets of material, resulting in low-frequency feed pulsations in the delivery of feed material to the burner, causing incomplete combustion. Such a system and the associated problem is described in Suenaga et al., “High-Performing Flash Smelting Furnace at Saganoseki Smelter & Refinery”, Second International Conference on Processing Materials for Properties, The Minerals, Metals & Materials Society, 2000, pages 879-884.
It is also well documented that known feed systems suffer from periodic flow instabilities associated with uncontrollable partial fluidization of the charge in the feed bins. This normally occurs during the charging cycle of the bin, and results in uncontrolled delivery of feed material to the burner, typically lasting between one and several minutes. This has negative consequences on all aspects of the combustion process.
While air-slides and alternative bin designs have been proposed to address the above issues, these approaches suffer from serious drawbacks: Air-slides are incapable of eliminating low-frequency feed pulsations, serving instead to transmit them to the burner. Alternative bin designs, typically with a mass-flow hopper, can reduce the severity of the flushing phenomenon, but are typically large, or severely decrease the capacity of the bin for a given bin height or footprint. This makes retrofit of the alternative bins into existing feed systems costly and impractical.
Another example of a typical feed problem faced by concentrate burners is poor distribution of feed around the circumference of the burner. Feed systems usually contain one or more feed pipes that interface with the injector and attempt to utilize splitter boxes, guides and diverter chutes to distribute feed evenly around the circumference. Such systems tend to cause the feed to gather at corners/edges of the chute walls and fins, forming dense, “ropes” of feed within the plume. This lack of spatial uniformity results in poor ignition characteristics, non-uniformity of the combustion plume and reduced oxygen efficiency.
Pneumatic conveying systems have been proposed in an attempt to resolve both the pulsation problems, but these require a large investment of capital for new equipment, as well as substantial modifications to existing building layouts to accommodate the system. These systems do not, however, eliminate the problem of non-uniform circumferential distribution of the feed at the burner inlet, because they feed through intermediate, feed chutes, splitters or other equipment, and deliver the feed through discrete points around the circumference of the burner, necessarily leading to a lack of uniformity.
The process disturbances caused by temporally and spatially non-uniform delivery of the feed to flash smelting burners represent a significant loss of economic value to the flash smelter operator. None of the existing technologies adequately solves the problem of feed delivery. There thus remains a need for feed systems for flash smelting furnaces, or other applications using a particulate feed material, which provide uniform flow, both spatially and in time, around an inlet annulus with minimum particle segregation effects and in which the feed rate can be accurately controlled.